1. Field of the Invention
The present invention relates generally to valves and, more particularly, to a fugitive emission leak detection system for the valve packing of a valve, including valves which use live loaded packing assemblies or systems.
2. Description of the Related Art
In a typical valve construction, a valve stem may undergo a turning or sliding movement, or a combination of both movements, within its sleeve during the process of the valve moving between its open and closed configurations. In this regard, the sealing of the stem must be adequate to contend with such movement, while at the same time ensuring maintenance of fluid tightness against the pressure of the fluid flowing through the valve. A widely used type of stem sealing is a compression packing in which a gland or sleeve is used to apply a compressive force to a compression packing which surrounds a portion of the length of the stem. The resulting radial pressure of the packing onto the stem provides the desired seal so long as the radial pressure exceeds the pressure of fluid in the valve.
In certain valve configurations, compression may be applied to the packing through the use of packing bolts which are each attached at one end to a valve bonnet of the valve, and at their other end to a spigot, a flange or other projection bearing on, integral with or attached to the gland or sleeve which bears onto the packing. In this particular arrangement, the tightening of the bolts increases the pressure on the packing, thus facilitating the application of radial pressure onto the stem.
In other valve configurations, it is known to attach a spring between the nut used to tighten the bolt and a surface of the spigot or flange. Although coil springs may be used, a conventional practice is to use Belleville springs which are essentially formed as a series of dished washers. Such springs have a higher compression rating than a simple coil spring, with the use of the Belleville springs providing a “live-loaded” packing which can automatically compensate for changes that may take place in the packing under operating conditions of the valve, such as high pressures and temperatures. Since the volume of the packing material may reduce under certain operating conditions, or the temperature increase of the bolts and their further elongation may result in a load loss, the spring pressure compensates for such reduction and maintains the required pressure, thus avoiding potential harmful effects to the sealing of the stem in an unsprung valve which could result from the reduction in the packing material volume. Alternatively, if the volume of the packing material increases (which can happen with certain packing materials), the radial pressure of the stem in an unsprung valve could increase too much, thus possibly causing sticking of the stem. The spring value, however, can accommodate the pressure increase by means of further compression of the springs.
In certain valve designs, including those which include a live-loaded packing as described above, the packing is segregated into a primary packing and a secondary packing. In these designs, only the primary packing should normally being pressurized by the fluid in the valve, the secondary packing working with a zero pressure drop on it. In a number of existing valves including both a primary and secondary packing, an inter-stage port or leak port extends through the valve bonnet of the valve into fluid communication with a passageway which extends through the valve bonnet and accommodates the valve stem. However, in current valve designs, this leak port is normally closed, and is only periodically opened manually to monitor the status of the primary packing. A major drawback of the leak port being normally closed is that any leak from the from the primary packing will pressurize the secondary packing, which typically results in both the primary and secondary packings becoming worn and compromising their operation, thus further destroying the efficacy of the secondary packing as a back-up to leakage through the primary packing.
Recently, there has been an increasing level of demand in many oil and gas applications for the low level emission of Volatile Organic Compounds (VOC's). In this regard, various laws enacted in Europe and other jurisdictions currently define the maximum concentration level of pollutants that can be detected in the air in an industrial setting, and proximate valves located therein. These laws and regulations are having the effect of forcing valve manufactures to adopt new designs for valve packing and sealing systems to comply with the same. These packing and sealing systems are tested during the design phase of the valve, and are again tested during the production phase thereof.
A current practice in oil and gas industrial settings is to use analyzers to monitor for potential leaks from valves and other fluid conduits which would giving rise to undesirable pollutant concentrations in the surrounding air. These analyzers are adapted to provide an alarm to the plant operators, but do not effectively provide a way to verify that the cause of such alarm is actually attributable to the leakage of a valve as opposed to the pollutant concentrations emanating from another source. These analyzers further do not provide any modality to immediately reduce the emission level of the pollutant into the atmosphere in the event such pollutant is actually emanating from a valve. As a result, in current practices involving the handling of hazardous fluids, the generation of an alarm by the analyzer will sometimes result in a complete shut-down of the plant, with the lack of any emission reducing capability also creating the potential for injury to plant workers present in the area of a valve leak. The present invention addresses this problem by providing a fugitive emission detection system which is particularly suited for use in conjunction with valves including primary and secondary packings, as is operative to continuously monitor the emissions from the valve into the atmosphere and to actively cut or reduce such emissions by effectively switching the operation of the valve sealing system from the primary packing to the secondary packing. These, as well as other features and attributes of the present invention will be discussed in more detail below.